1. Field of the Invention
The present invention relates to an information compression method and device for compatible decoding of a group of television signals with increasing resolutions. It applies more particularly to digital video-signal transmitting and receiving systems and to digital video recorders.
2. Description of the Prior Art
As is well-known in the art, to reduce the information flow in information compression devices, the digitized image is coded by using a cosine-type two-dimensional transformation, according to Fourier, Hadamard, Haar, or Karhunen-Loeve. Transform coding enables the use of the statistical properties of the image source and the observers' psychovisual aptitude. As the number of operations required to obtain these codings increases rapidly in relation to the size of the image, the latter is divided into windows or blocks and the transformation is applied to each of the window pixels. This two-dimensional transformation means that the same number of coefficients are obtained as there are pixels in each block of images.
Flow reduction is obtained by quantifying the coefficients and by describing the sequence of obtained values with the assistance of a variable length code. Due to the fact that this final operation produces a variable flow, and that the television channel has a steady flow, a buffer memory is added to the coder output to guarantee a regular flow of information through the coder.
Decoding consists in deciphering variable-length received code words in order to discover the original two-dimensional transformation coefficients, then, in restoring the digital values for each image element, by applying to the transformation coefficients corresponding to each image block, the inverse two-dimensional transformation of the one originally used for coding. The transformations which are used in practice are those for which there exist rapidly performed algorithms, as is the case, for example, with cosine-transformation coding algorithms, for which descriptions may be found in French patent applications No. 2575351 or 8718371 registered under the name of the Applicant. Generally speaking, such systems can be applied to images for which the resolution is known in advance, with the number of lines and points per line previously defined, as for example 720 points for 576 lines, defined in the CCIR recommendation 661. But, in parallel to this type of resolution, there exist other types of resolution, such as those of high-definition television, described, for example, in the article by J. CHATEL entitled "Compatible Hierarchy of Studio standards" Conf. SMPTE San Francisco Feb. 1-3, 1989 and reduced 288-line, 360-point resolution for applications requiring a lower-quality image, as for example, the videophone. This diversity of resolutions implies a diversity in both coding and decoding systems as well as complications in the management of satellite or radio channel links which have, in some cases, to retransmit as many audio-visual programs as there are systems. It also imposes restrictions on users who may need to change TV receivers in order to benefit from the image quality offered by high-definition television.